The present invention is directed to improved polybutadiene compositions for use in molded golf ball core construction. The improved polybutadiene compositions contain polypropylene powder resins, which have the effect of increasing the hardness of the resulting molded cores without increasing the weight and/or substantially reducing the resiliency of the ball. The present invention is also directed to golf balls produced utilizing the improved core compositions.
Moreover, it has been found that the addition of the particulate polypropylene allows for a reduction in the amount of crosslinking agents utilized in the core compositions and/or an increase in the amount of higher specific gravity fillers, thereby dramatically decreasing the cost of the golf ball cores. A method for producing such an effect is also disclosed.
Two of the principal properties involved in the performance of golf balls are resilience and hardness. Resilience is determined by the coefficient of restitution (C.O.R.), the constant "e", which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact, or more generally, the ratio of the outgoing velocity to incoming velocity of a rebounding ball. As a result, the coefficient of restitution (i.e. "e") can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision. Hardness is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball's diameter (i.e. the lower the compression value, the harder the material).
Resilience (C.O.R.), along with additional factors such as clubhead speed, angle of trajectory, and ball configuration (i.e. dimple pattern), generally determines the distance a ball will travel when hit. Since clubhead speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
In this regard, the coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and measuring the ball's incoming and outgoing velocity electronically. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golfers Association (U.S.G.A.). Along this line, the U.S.G.A. standards indicate that a "regulation" ball cannot have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second (250 feet per second with a 2% tolerance). Since the coefficient of restitution of a ball is related to the ball's initial velocity (i.e. as the C.O.R. of a ball is increased, the ball's initial velocity will also increase), it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of hardness (i.e. impact resistance) to produce enhanced durability.
The coefficient of restitution (C.O.R.) in solid core balls is a function of the composition of the molded core and of the cover. In balls containing a wound core (i.e. balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings.
The present invention relates to the addition of a polypropylene powder resin to polybutadiene core compositions, such as those disclosed in U.S. Pat. Nos. 4,726,590; 4,838,556; 4,844,471; and 4,852,884 in order to produce solid golf ball cores and/or centers exhibiting enhanced hardness without an increase in weight and a substantial reduction in C.O.R. It has been found that the addition of a polypropylene powder resin results in a core which is too hard (i.e. exhibits low compression) and thus allows for a reduction in the amount of crosslinking agent utilized to soften the core to a normal compression.
Furthermore, because polypropylene powder resin can be added to core composition without an increase in weight of the molded core upon curing, the addition of the polypropylene powder allows for the addition of large amounts of higher specific gravity fillers, such as mineral fillers. Since the crosslinking agents utilized in the polybutadiene core compositions are expensive and/or the higher specific gravity fillers are relatively inexpensive, the addition of the polypropylene powder resin substantially lowers the cost of the golf ball cores while maintain proper weight and compression.
Along these lines, although other particulate thermoplastic materials have been evaluated, including relatively low specific gravity thermoplastic powders (i.e. polystyrene, polyethylene, polymethyl methacrylate, etc.), it has been found that these powdered resins do not increase the hardness of the cores to the degree desired, thus still requiring use of larger amounts of the expensive crosslinking agents such as zinc diacrylate, and/or they reduce the C.O.R. values to undesirable ranges. Furthermore, the alternative thermoplastic powder resins produce, when compounded, heavier cores than the present invention. Consequently, the addition of the inexpensive mineral fillers is substantially limited in the powdered plastics in comparison to the present invention.
Accordingly, it is an object of the present invention to provide improved polybutadiene core compositions which, when utilized to formulate golf balls, produce golf balls exhibiting enhanced hardness at a lower weight due to the composition of the core. An additional object of the invention is to provide cores or centers for the construction of solid or wound golf balls which dramatically lower the cost of the golf ball while maintaining proper weight and compression. Furthermore, since a golf ball produced in accordance with the present invention exhibits an enhanced hardness at a lower weight, it is possible to increase the durability of the ball by adding additional durability enhancers without greatly sacrificing the ball's overall weight and/or composition. A still further object of the invention is to produce golf ball cores or centers which maintain their roundness and shape upon continued impact by a golf club.
These and other objects and features of the invention will be apparent from the following summary and description of the invention and from the claims.